1. Field of the Invention
This invention relates to apparatus for monitoring and detecting a pending failure, operation of and/or inoperability of a heart pacemaker, and in particular, to apparatus for monitoring the voltage level of the pacemaker power supply, initiation of heart stimulation and/or the impedance presented to the output of the pacemaker.
2. Description of the Prior Art
Heart pacemakers such as that described in U.S. Pat. No. 3,057,356, issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulus to the heart whereby it is contracted at a desired rate in the order of 72 beats per minute. A heart pacemaker is capable of being implanted in the human body and operative in such an environment for relatively long periods of time, to provide cardiac stimulation at relatively low power levels by utilizing a small, completely implanted, transistorized, battery-operated pacemaker connected via flexible electrode wires directly to the myocardium or heart muscle. The electrical stimulation by this pacemaker is provided at a relatively fixed rate.
Such cardiac pacemakers of the implantable variety have found wide acceptance for heart patients suffering from complete heart block and other defects of normal conduction. As a result, the use of these pacemakers has increased the life expectancy of those patients with implants, from a 50% probability of 1 year to nearly the life expectancy of physically-comparable humans not suffering from the same heart disorder.
Typically, such cardiac pacemakers are encapsulated in a substance substantially inert to body fluids, and are implanted within the patient's body by a surgical procedure wherein an incision is made in the chest beneath the patient's skin and above the pectoral muscles or in the abdominal region, and the pacemaker is implanted therein. Due to the inconvenience, expense and relative risk to the patient's health, it is highly desired to extend the life of the power source or battery, whereby the number of such surgical procedures is limited. The resultant problem for the attendant doctor is to determine when the batteries should be replaced, keeping in mind the relative risk or probability of premature pacemaker failure due to battery depletion.
A number of solutions to this problem have been proposed, one being replacement at predetermined intervals, thus accepting an empirically determined risk or failure of the pacemaker batteries. Another proposed solution is to establish pacemaker "clinics" where photographic analysis techniques are used to detect imminent failure. These solutions are not entirely satisfactory for detecting, simply and positively, a degradation of pacemaker system performance. The risk of undetected premature failure associated with periodic replacement at predetermined intervals is obviously undesirable. Photoanalysis techniques are complicated, not positive in detection and require that the patient be physically present in the physician's office. Further, such techniques are not readily available to physician and patient on short notice, but rather, as mentioned previously, would be available only at special clinics, and, furthermore, are scheduled at relatively infrequent intervals.
In U.S. Pat. No. 3,618,615, assigned to the assignee of this invention, there is disclosed an artificial cardiac pacemaker for generating at regular intervals a train of stimulating pulses, one of which is of significantly lower energy than the other pulses. If the heart responds to the reduced energy stimulating pulse, an adequate safety factor remains, but if the heart does not respond, e.g. no beat is detected in response to the lower energy or test pulse, marginal operation and possible imminent failure is ascertained.
Another method of ascertaining the pending failure of a pacemaker energy source is described in U.S. Pat. No. 3,713,449, assigned to the assignee of this invention, describing an artificial pacemaker including means for varying selectively the pulsewidth of its stimulating pulse. Control of the pulsewidth is made preferably by a mechanism external of the body by an attending physician. By such mechanism, the physician varies the pulsewidth of the implanted pacemaker until capture is lost. As the physician has previously measured the pulsewidth at the time of pacemaker implant, the pulsewidth at a subsequent time may be varied until capture is lost, whereby the state of the battery can be determined with respect to its replacement.
In an alternative approach to the problem of accurately determining battery depletion, there are artificial pacemakers such as described in U.S. Pat. No. 3,842,844, having a battery or cell depletion indicator that increases the pulsewidth of the output signal as their batteries deplete, i.e., their voltage amplitude decreases. Further, as the power source or battery depletes, the pulse repetition rate of such artificial cardiac pacemakers also decreases. For example, at the time of implantation, an artificial cardiac pacemaker may produce stimulating pulses at 70 pulses per minute (PPM), plus or minus two beats, with a pulsewidth in the order of 0.5 msec. After a period of service illustratively in the order of 2-4 years, the PPM changes in the order of 5%-10%, i.e., a decrease of 5-7 beats from the original PPM, and the pulsewidth may increase to a value in the order of 1 msec. Dependent upon the known histories of such batteries, such a change in the pulse rate as well as a change in pulsewidth indicates that one of a plurality of (e.g. 4 or 5) cells has failed, and that it is time to replace the batteries within the implanted pacemaker to assure continued heart stimulation of a sufficient level.
Further, in the prior art, there is known a still further method for detecting and providing a manifestation indicative of the dissipation or pending failure of a heart pacemaker power supply. A primary and an auxiliary power source are used in conjunction with a diode switching circuit, which responds when the voltage level of the primary source becomes less than that of the secondary source, to apply the secondary source to a timing pulse generator. Due to the voltage characteristics of the diodes incorporated within the switching circuit, a slightly reduced voltage is applied by the secondary source to the generator, whereby the generator changes slightly its stimulus pulse rate. It appears that the slight change in pulse rate would not normally be sensed by the patient within whom such a heart pacemaker is implanted, but would require a physician to measure the pulse rate to detect a pending depletion or failure of the heart pacemaker power supply. In a further disclosed embodiment, a second set of electrodes including an indifferent electrode plate and a second electrode are disposed in body tissue remote from the heart, whereby when the voltage level of the primary power source falls below that of the auxiliary power source, the auxiliary power source is applied to the second set of electrodes whereby the body tissue between the second set of electrodes is stimulated so that the patient is made directly aware of the possibility of the failure of the heart pacemaker power source. In such an arrangement, there is required a redundant power source and there are no convenient means for de-energizing the second set of electrodes, whereby the auxiliary power source is required to energize not only the pulse generator, but also the second set of electrodes. As a result of this double load, the auxiliary power is drained fairly rapidly. Further, the disclosed excitation of the second set of electrodes would be by a DC-type battery source, providing a steady level of stimulation to the patient. In some instances, it is contemplated that the patient would be more sensitive or responsive to a modulated stimulation than that provided by this pacemaker.
Further, it is desirable to provide a warning that the pacemaker leads have failed or are about to fail. As discussed above, a pacemaker is implanted within the human body by a surgical procedure, with at least one of its leads interconnected between the pacemaker and the patient's heart. As the patient moves in terms of bending over or stretching with the pacemaker implanted within his body, the pacemaker lead will be bent continuously to the point where the conductors thereof break or split or the insulating material of the pacemaker leads deteriorates. Further, it is contemplated that during the implantation of the pacemaker, the insulating material of one of the leads may be cut, thus exposing a portion of the electrical conductor. If the pacemaker leads have failed or are about to fail, the level of stimulation to the patient's heart is reduced significantly, if not blocked altogether. Therefore, it is desirable to provide a detector or monitor circuit for detecting the impedance and to provide the patient with an indication thereof.
Further, in those patients who receive a permanently-implanted pacemaker of the "demand" type in anticipation of imminent sudden disruption of the normal conduction sequence of the heart, it is desirable to provide a warning that the pacemaker has begun to operate and is stimulating the heart. Such a warning is of particular importance for patients who have received a pacemaker prophylactically, the pacemaker being in a dormant state at the time of implantation and in the immediate period following implantation, and due to a potentially life-threatening change in the condition of the heart's conduction system, the pacemaker has begun to stimulate the heart. Such patients, upon receiving the warning, would be encouraged to seek professional (i.e., medical) assistance to assess their immediate condition.